Storm Forecast
Valid: Tue 04 Sep 2007 06:00 to Wed 05 Sep 2007 06:00 UTC
Issued: Tue 04 Sep 2007 10:09
Forecaster: VAN DER VELDE

SYNOPSIS

A mid/upper level low initially over southern Scandinavia sinks into a longwave trough over central Europe and finally causes a large stationary cut-off upper low with QG motions spanning 20-30 degrees of longitude/latitude. High sea level presssure builds over western Europe, and a low deepens over the Balkan.


DISCUSSION

Italy, Adriatic coast:

Very strong forcing along the cold front in unstable airmass has created convection looking currently like a squall line over Italy and a possible supercell over Croatia. While 0-6 km shear is maximized over the cold side of the cold front (later improving SE of the front), 0-3 km SREH and 0-1 km shear can locally reach critical amounts (10-12 m/s) for mesocyclones and tornadogenesis. Low LCL heights (<1000 m or even <500 m, evening) and strong low level buoyancy advected from the warm (>24 degrees) Adriatic Sea are in support of this.
Large hail should occur in several places.
Large precipitation amounts are forecast by GFS and may cause flash floods in the level 2 region and eastward. As the cold front slowly moves southeastward, the threat shifts towards Albania and Greece by the end of the period.

As the low level vertical temperature gradient increases over the warm Adriatic, waterspouts may be sighted, also and perhaps especially behind the cold front where convection eventually returns over sea (Tsfc-T500m >25 degrees!)... in the early morning of Wednesday winds blow onshore to the eastern Italian coast with there the best chance of convection.


Turkey:

Persisting situation of the past few days still allows convection today... 1000 J/kg CAPE positioning in >20 m/s deep layer shear with somewhat helical inflow (200 m2/s2 SREH3) suggests supercells with large hail are the likely mode, GFS delta-theta-e suggests very dry mid levels for enhanced downburst potential (severe gusts).


SE Germany:

During the evening this is where the CAPE max is forecast by GFS, and >10 m/s low level shear and marginal (100-200 m2/s2) SREH are in place... which poses a marginal threat of small mesocyclones causing tornados. However, diurnal heating will fall and convection may stop altogether, perhaps out of range from QG lift from the upper shortwave coming from the north.


Baltic Sea coastal areas:

Chance of spouts appears to exist, given weak flow (slow moving convection) and steep lower lapse rates, good 0-3 km CAPE (even better in some 00Z soundings) and moist profiles. Coastal convergence zones or overnight landbreeze convergence zones (Finnish Gulf) may provide vertical vorticity.